The present invention relates to a compact size antenna device used in, for example, a communication system having a wide bandwidth or a communication system for commonly using two or more communication systems. Particularly, the present invention relates to an antenna device having two resonance frequencies.
FIG. 1 and 2 are diagrams showing prior art antennas. FIG. 1 shows a printed antenna having two radiating patches disposed in opposing relation to each other. FIG. 2 shows a printed antenna having two radiating patches disposed laterally to each other in a common plane. Here, reference numerals 101A, 101B denote radiating patches composed of two conductor plates having different lengths or widths from one another. A reference numeral 102 denotes a feeder line, 103 is a short-circuit metal plate extending between the radiating patches 101A, 101B and a ground plate 104, and 120 is a dielectric plate. In this way, a prior art antenna device attempts to provide two resonance frequencies or a wide bandwidth on a single antenna by resonating the two different sized radiating patches at two different frequencies.
In this case, if a ratio of the two resonance frequencies F.sub.L and F.sub.H is larger than approximately 1.5 (i.e., 1.5 F.sub.L &lt;F.sub.H), it is relatively easy to materialize such an antenna device. However, it is difficult to resonate the antenna at two close frequencies having a ratio therebetween less than approximately 1.5 (F.sub.L &lt;F.sub.H &lt;1.5F.sub.L), or to attempt a wide bandwidth by using two close frequencies. This is because, since the two resonance wavelengths are close to each other and the two radiating patches are disposed very closely, electromagnetic coupling between the two radiating patches becomes large and the two radiating patches can be regarded as an electrically single body, thus diminishing the effect of using two radiating patches. This problem is significant in the case in which two radiating patches are disposed on upper and lower sides of the dielectric plate 120 as shown in FIG. 1. However, this phenomenon is also significant in the antenna shown in FIG. 2.
Further, since the space between two radiating patches must be large to suppress this problem, there is a shortcoming that the size of antenna becomes large. On the other hand, if, in a state where electromagnetic coupling between the radiating patches is large, the antenna is forced to resonate at two close frequencies by using a matching circuit etc., there would also be a shortcoming that a loss in the matching circuit increases, and thus antenna gain is reduced.
Therefore, in a conventional antenna, there are the following shortcomings: (a) When two radiating patches are disposed very closely, the electromagnetic coupling between them is very large and thus the antenna cannot be resonated at any two desired frequencies; (b) When the antenna is resonated at two very close frequencies, or when a wide bandwidth is attempted by making those two frequencies much closer, the size of antenna becomes large since the space between the radiating patches must be large in order to reduce electromagnetic coupling between the radiating patches; (c) When the space between the radiating patches is made small and the antenna is forced to resonate at two close frequencies by a matching circuit etc., the antenna gain is reduced.